The superior colliculus is a key center related to the initiation of rapid or saccadic eye movements, and one of the major influences acting on it is from the pars reticulata of substantia nigra. This segment of the substantia nigra is one of the two major output pathways of the basal ganglia. Our previous experiments have suggested that this pathway produces a tonic inhibition on the superior colliculus which is released at the time of saccade initiation. In the first experiment described, we tested this hypothesis by injecting a transmitter agonist gamma;amino butyric acid (GABA), into the substantia nigra of an awake, behaving monkey and determined the effect on the monkey's ability to initiate saccadic eye movements. The injections produced irrepressible saccades to the part of the visual field served by substantia nigra. The monkey had greater difficulty making saccades to remembered targets rather than to visual targets. The results of these experiments are most easily interpreted as an increase of inhibition acting on the substantia nigra, a decrease in rate of discharges of substantia nigra cells, and a release of the superior colliculus from tonic inhibition. In a second set of experiments, the eye movements in patients with Parkison's disease were examined since this disease affects the basal ganglia and might be expected to change the output related to the generation of saccadic eye movements. We found that the patients, like the monkeys, had greater difficulty making saccades to a remembered location than to a visual target. While the final eye position was nearly the same in both conditions, the velocity of the saccades to the remembered location was markedly altered, particularly during deceleration. This result, combined with the relatively normal saccades made to visible targets, suggests that in man, as in monkey, the basal ganglia are an important influence on saccadic control, particularly to rembered targets. In Parkingon's disease, the tonic inhibition of the substantia nigra on the superior colliculus presumably cannot be completyely released to allow a normal saccade.